1. Field of the Invention
The present invention relates to a vibration wave motor which drives a movable member by a travelling vibration wave, and more particularly to a vibration direction of an electrostrictive element for generating the vibration wave.
2. Description of the Prior Art
As disclosed in U.S. Pat. No. 4,019,073, the vibration wave motor translates a vibration motion created when a periodic voltage such as an A.C. or a pulsating voltage is applied to an electrostrictive element to cause a rotating motion or a linear motion. Because it requires no winding as opposed to a conventional motor, it is simple and small in structure and can provide a high torque at a low rotating speed and has a low moment of inertia.
In the vibration wave motor disclosed in U.S. Pat. No. 4,019,073, the movable member such as a rotor which contacts the vibration member is friction-driven in one direction by a standing vibration wave created in the vibration member to translate the vibration motion to the rotating motion. During a forward motion of the vibration, the vibration member makes frictional contact with the movable member, and during a backward motion, they separate from each other. Accordingly, the vibration member and the movable member must make contact in a small area, that is, a point contact or a line contact. As a result, the friction drive efficiency is low.
A vibration wave motor which improves the above aspect and friction-drives the movable member by a travelling vibration wave created in the vibration member is desirable.
FIG. 1 shows a schematic view thereof. Numeral 1 denotes an electrostrictive element such as PZT (e.g. the solid-solution of PbZrO.sub.3 and PbTiO.sub.3) and numeral 2 denotes a vibration member made of an elastic material to which the electrostrictive element 1 is bonded. The vibration member 2 and the electrostrictive element 1 are held on a stator (not shown). Numeral 3 denotes a vibration member which is press-contacted to the vibration member 2 to form a rotor.
FIG. 2 is a side view showing a relation between the electrostrictive element 1 and the vibration member 2. The electrostrictive element 1 includes a plurality of elements 1a.sub.1, 1a.sub.2, 1a.sub.3, . . . and 1b.sub.1, 1b.sub.2, 1b.sub.3, . . . and one group of elements 1a.sub.1, 1a.sub.2, 1a.sub.3, . . . are arranged to shift by one quarter of a wavelength .lambda. of the vibration wave from the other group of elements 1b.sub.1, 1b.sub.2, 1b.sub.3, . . . . In the one group of elements, the elements 1a.sub.1, 1a.sub.2, 1a.sub.3, . . . are arranged at a pitch of one half of the wavelength with opposite polarization polarities between adjacent elements. In FIG. 2, (+) and (-) indicate the polarities. In the other group of elements, the elements 1b.sub.1, 1b.sub.2, 1b.sub.3, . . . are arranged at the pitch of one half of the wavelength with the opposite polarities between adjacent elements. Alternatively, a single electrostrictive element having the same size as that of the arrangement of the elements 1a.sub.1, 1a.sub.2, . . . , 1b.sub.1, 1b.sub.2, . . . may be polarized at the pitch of one half of the wavelength. Electrodes for applying voltages to the electrostrictive elements are vapor-deposited or formed on both surfaces of the electrostrictive element.
In this vibration wave motor, an A.C. voltage of V.sub.0 sin .omega.T is applied to all electrostrictive elements 1a.sub.1, 1a.sub.2, 1a.sub.3, 1a.sub.4, . . . in one group, and an A.C. voltage of V.sub.0 cos .omega.T is applied to the elements 1b.sub.1, 1b.sub.2, 1b.sub.3, 1b.sub.4, . . . of the other group. Thus, A.C. voltages which are phase-shifted by 180 degrees from each other between adjacent ones and by 90 degrees between two groups are applied to the electrostrictive elements so that they expand and shrink. This vibration is propagated to the vibration member 2, which is bent in accordacne with the pitch of the arrangement of the electrostrictive elements 1. The vibration member 2 protrudes at every other electrostrictive element position and sinks at every other alternate position. As described above, since one group of the electrostrictive elements is one quarter of the wavelength shifted from the other group, and the phases of the bending vibrations have 90 degrees phase difference from each other, the vibration waves are combined and travel. While the A.C. voltages are applied, the vibrations are successively excited to cause travelling bending vibration waves, which propagate through the vibration member 2.
The motion of the wave is illustrated in FIGS. 3(a)-(d). Assuming that the travelling bending vibration wave travels in an X direction and 0 denotes a center plane of the vibration member in a quiscent state, in a vibration state, a neutral plane 6 shown by a chain line is acted on by bending stresses. Considering a sectional plane 7 normal to the neutral plane 6, no stress is applied to a crossing line 5 of those planes and it merely vibrates vertically. The sectional plane 7 makes a pendulum vibration laterally around the crossing line 5. In FIG. 3(a), a point P on a crossing line of the sectional plane 7 and the surface of the vibration member 2 facing the movable member 1 is a right dead center of the lateral vibration and makes only an upward motion. In this pendulum vibration, a leftward stress (opposite to the wave travel) is applied when the crossing line 5 is on a positive side of the wave (above the center plane 0), and a rightward stress is applied when the line 5 is on a negative side of the wave (below the center plane 0). In FIG. 3(a), a crossing line 5' and a sectional plane 7' correspond to the former case in which a stress F' is applied to the point P, and a crossing line 5" and a sectional plane 7" correspond to the latter case in which a stress F" is applied to the point P. As the wave travels and the point P comes to the positive side of the wave as shown in FIG. 3(b), the point P makes a leftward motion and an upward motion simultaneously. In FIG. 3(c), the point P is a top dead center of the vertical motion and makes only the leftward motion. In FIG. 3(d), the point P makes the leftward motion and the downward motion. As the wave further propagates, the point P makes the rightward and downward motions, and the rightward and upward motions, and returns to the state of FIG. 3(a). By the combination of the series of motions, the point P makes a rotating elliptic motion. A radius of rotation is a function of t/2 where t is a thickness of the vibration member 2. As shown in FIG. 3(c), on a line at which the point P contacts the movable member 3, the movable member 3 is friction-driven in an X' direction by the motion of the point P.
The vibration wave motor driven in this manner does not provide sufficient drive efficiency.
The manner of the vibration of the electrostrictive element is now explained in further detail. In FIG. 4A, when a positive (forward direction H.sub.A) voltage is applied to the electrostrictive element 1a.sub.2 in the polarization direction from an A.C. drive source 9, the electrostrictive element 1a.sub.2 expands in the electric field direction, that is, the thickness direction and shrinks in the direction normal to the electric field direction, as shown by arrows A. In FIG. 4B, when the voltage is applied to the electrostrictive element 1a.sub.2 in the opposite direction H.sub.B, it shrinks in the electric field direction and expands in the direction normal to the electric field direction as shown by arrows B. As the electrostrictive element 1 expands and shrinks in the direction of the bonding surface to the vibration member 2, the vibration member 2 bonded thereto is bent.
The direction of the voltage applied to the electrostrictive element 1 is normal to the direction of the expansion/shrinkage motion which causes the bending vibration in the vibration member 2. That is, the direction of the voltage application and the direction of the expansion/shrinkage motion cross each other to make a lateral effect motion. Because of the lateral effect motion, the drive efficiency of the prior art vibration wave motor is low.